Digital television programs globally increase. Demands of digital televisions are inevitably expanding day by day. Current digital televisions are divided into two main-stream categories, namely the European Digital Video Broadcasting (DVB) standard, and the American ATSC standard (ATSC). The American digital television specifications have an especially defined captioning standard, Digital Television Closed Captioning (DTVCC), for hearing-impaired individuals. Under a mute condition, apart from showing dialogues between characters, the DTVCC also displays descriptive sentences on events occurring on the screen. For example, descriptive sentences are displayed when there are background sounds on the screen to help hearing-impaired individuals to better understand the plot.
Video signals containing closed captioning, sent from a head-end of a DTV or outputted from a DVD player by accessing a DVD, along with a window-defining command for defining the DTVCC, are decoded by a built-in captioning decoder in an ATSC digital television to display the closed captioning. The image signals along with the captioning are then displayed on a display panel. In the prior art, a captioning decoder is generally achieved using a specialized independent chip.
The DTVCC specifications allow a maximum of eight window-defining commands DF0 to DF7 in a video signal from a DTV head-end or a DVD player. More specifically, when being fully supported by a DTV system, a DTV display is able to simultaneously display eight caption windows. However, in practical applications, it is rarely that more than three caption windows are displayed at the same time, so that a standard of simultaneously displaying only four caption windows is provided in a DTV of minimum function requirements. That is, in some DTVs, certain caption windows shall not be displayed when the video signal contains more than four caption windows.
According to the DTVCC specifications, a window-defining command is defined with numerous fields as shown in FIG. 1. For example, a window ID field is filled with values from 0 to 7; and a priority field representing a display priority of caption windows is similarly filled with values from 0 to 7, where 0 has the highest priority and 7 has the least priority. Accordingly, the DTV determines caption windows not to be displayed based on the values in the priority field. Nevertheless, priorities of multiple caption windows sent from a head-end system are generally set as the highest; thus, the prior art may not effectively determine which caption windows should not be displayed and result in errors in displaying the caption windows. It is therefore an objective of the invention to overcome the various shortcomings of the prior art.